Filamentous fungi and cellulolytic bacteria produce extracellular cellulase enzymes that confer on the organisms the ability to hydrolyze the β-(1,4)-linked glycosidic bonds of cellulose to produce glucose. These enzymes provide the organisms with the ability to use cellulose, the most abundant plant polysaccharide, for growth.
The filamentous fungus, Trichoderma reesei, is an efficient producer of cellulase enzymes. As such Trichoderma reesei has been exploited for its ability to produce these enzymes, which are valuable in the production of such commodities as fuel ethanol, clothing, detergents, fibers and other products.
The cellulolytic mix of Trichoderma reesei proteins is among the best characterized cellulolytic pathways of microorganisms. The cellulases that comprise these mixes are classified into two broad categories: the endoglucanases (EG) and the cellobiohydrolases (CBH). β-glucosidase is also part of the cellulase mix of Trichoderma reesei. 
Trichoderma reesei has also been exploited for its ability to produce heterologous proteins. Genes encoding a desired protein can be regulated when they are operably linked to the inducible cbh1 promoter of T. reesei. Foreign polypeptides have been secreted in Trichoderma reesei as fusions with the catalytic domain plus linker region of cbh1 (Nyyssonen et al., Bio/technology 11:591-595, 1993).
Expression of the genes comprising the cellulase system is coordinate and regulated at the transcriptional level. The members of this system act synergistically, and as noted above, are necessary for the efficient hydrolysis of cellulose to soluble oligosaccharides.
Expression and production of the main cellulase genes in Trichoderma, cbh1, cbh2, egl1, and egl2, is dependent on the carbon source available for growth. The cellulase genes are tightly repressed by glucose and are induced several thousand fold by cellulose or the disaccharide sophorose. Indeed, the expression level of the major cellobiohydrolase 1 (cbh1) is up-regulated several thousand fold on media containing inducing carbon sources such as cellulose or sophorose compared with glucose containing media (Ilmen et al., App. Environ. Microbio., 1298-1306, 1997).
Commercial scale production of cellulase enzymes is by either solid or submerged culture including batch, fed batch, and continuous flow processes. The most problematic and expensive aspect of industrial cellulase production is providing the appropriate inducer to Trichoderma. As is the case for laboratory scale experiments, cellulase production on a commercial scale is induced by growing the fungus on solid cellulose or by culturing the organism in the presence of a disaccharide inducer such as lactose. Unfortunately on an industrial scale, both methods of induction have drawbacks which result in high costs being associated with cellulase production.
Cellulase synthesis is subject to both cellulose induction and glucose repression. Thus, a critical factor influencing the yield of cellulase enzymes or heterologous proteins under the control of an inducible promoter is the maintenance of a proper balance between cellulose substrate and glucose concentration; it is critical for obtaining reasonable commercial yields of the regulated gene product. Although cellulose is an effective and inexpensive inducer, controlling the glucose concentration when Trichoderma is grown on solid cellulose can be problematic. At low concentrations of cellulose, glucose production may be too slow to meet the metabolic needs of active cell growth and function. On the other hand, cellulase synthesis can be halted by glucose repression when glucose generation is faster than consumption. Thus, expensive process control schemes are required to provide slow substrate addition and monitoring of glucose concentration (Ju and Afolabi, Biotechnol. Prog., 91-97, 1999). Moreover, the slow continuous delivery of substrate can be difficult to achieve as a result of the solid nature of the cellulosic materials.
Allen and Mortensen (Biotechnol. Bioeng., 2641-45,1981) have shown that 200 IU/ml of β-glucosidase from Aspergillus phoenicis when incubated with a 50% glucose syrup produces a solution with the ability to induce cellulase production when used as a carbon source. These authors used more than 20× the β-glucosidase loading compared to that used in this current work.
Some of the problems associated with the use of cellulose as an inducing substrate can be overcome through the use of soluble substrates and inducers such as lactose or sophorose. Lactose has to be provided at high concentrations so as to function as an inducer and a carbon source. (See Seiboth, et. al., Mol. Genet. Genomics, 124-32, 2002.) Gentiobiose may also serve as an inducer. Sophorose is a more potent inducer than cellulose, but sophorose is expensive and difficult to manufacture. Thus, while it is easier to handle and control than solid cellulose, sophorose can nonetheless make the cost of producing cellulases prohibitively expensive and, thus, is impractical for commercial cellulase production. Clearly, a need exists for a convenient, soluble substrate composition that also provides an inexpensive method of cellulase induction in filamentous fungi, e.g., Trichoderma reesei. 
In addition, the ability to regulate inducible promoters to express either endogenous or heterologous genes with an inexpensive inducing agent would be of great commercial benefit.